The present invention relates to the art of liquid stock deaeration and supply. More particularly, the present invention relates to apparatus and methods for providing a flow of deaerated stock to a processing machine.
The manufacturing processes utilized in various industries employ liquid stocks or feed materials. For example, in the papermaking industry, suspensions of cellulosic fibers in water are employed as feedstocks for papermaking machines. As air entrained in such a feedstock may cause defects in the finished product and may impair the efficiency of the processing operation, various devices have been utilized for deaerating liquid stock before it is fed to a processing machine. Such devices generally employ an enclosed receiver and means for maintaining such receiver under a vacuum. The stock is introduced into the receiver and exposed to the vacuum to remove air from it.
The stock may be introduced into the vacuum receiver via spray pipes extending upwardly through the bottom wall of the receiver. In this arrangement, the stock is projected upwardly within the empty space or headspace in the receiver and impinges upon the top wall of the receiver to form finely divided droplets, thus intimately exposing the stock to the vacuum in the receiver for effective deaeration. The droplets fall to the bottom of the receiver where the deaerated stock is collected. Such an arrangement of spray pipes may be employed in a system where the stock is cleaned by hydrocyclones prior to deaeration. A large number of individual hydrocyclones may be disposed beneath the vacuum receiver and the "accept outlet" or clean stock outlet of each such hydrocyclone may be connected to one of the spray pipes.
In other arrangements, the stock to be deaerated may be introduced into the receiver at the top and directed so that it flows downwardly along the interior walls of the receiver in the form of a thin film, thus exposing it to the vacuum in the headspace. Regardless of whether the spray or film arrangement is utilized, the headspace space within the receiver should be large enough that the falling stock will be exposed to the vacuum for a long enough time to permit adequate deaeration.
The deaerated stock is ordinarily conveyed from the vacuum receiver to the processing machine via appropriate piping. The vacuum receiver is often mounted in an elevated location above the processing machine. In this arrangement, the "dropleg" or pipe extending downwardly from the receiver is filled with stock. Gravitational head or pressure of the stock in such pipe counteracts the vacuum or negative gauge pressure in the receiver, and thus assists flow of deaerated stock from the receiver. Also, mounting of the vacuum receiver in an elevated location conserves floor space in the mill.
A pump may be interposed in the piping between the vacuum receiver and the processing machine. Whether or not a pump is utilized, fluctuations in the conditions prevailing within the vacuum deaeration system may produce fluctuations in the stock pressure at the processing machine. For example, fluctuations in the level of stock in the vacuum receiver or in the piping connecting the receiver to the processing machine will induce corresponding fluctuations in the head or pressure of the stock at the processing machine. If a fluctuating vortex forms at the outlet of the vacuum receiver, such vortex may produce additional pressure fluctuations at the processing machine.
Fluctuations in stock pressure may in turn cause fluctuations in the rate of stock flow into the processing machine, and thus may produce undesirable variations in the operation of the machine. For example, fluctuations in the stock pressure and stock flow rate into a conventional papermaking machine will generally induce undesirable non-uniformity of weight, thickness and strength in the finished paper.
U.S Pat. 3,206,917 issued Sept. 21, 1965 to R. G. Kaiser, et al. describes a system for maintaining substantially constant stock pressure and flow to prevent such inefficiencies. The system disclosed in the Kaiser '917 patent employs a weir within the vacuum receiver, such weir subdividing the receiver into a main portion and an overflow portion. An outlet in the bottom of the main portion is connected to the processing machine. Stock is introduced into the main portion of the receiver and deaerated therein at a rate greater than the rate of stock withdrawal through the bottom outlet, so that a pond of deaerated stock accumulates in the bottom of the main portion until its level reaches the top of the weir, whereupon the excess stock overflows the weir. As long as the rate of deaerated stock production is greater than the rate of stock withdrawal through the main bottom outlet, excess stock will continually overflow the weir and the level of the pond in the main portion of the receiver will thus remain substantially constant, such level being substantially equal to the level of the top or crest of the weir. Such constant-level pond provides a constant stock pressure. The excess stock entering the overflow portion of the receiver is removed from the overflow portion by a separate recycle line and passed back to the stock supply for reintroduction into the receiver along with other stock to be deaerated.
Because this system economically provides both effective deaeration and effective protection against stock pressure fluctuations, it has been widely adopted by the papermaking industry. The present invention, however, incorporates recognition of certain opportunities for even further improvement.
In apparatus acccording to the Kaiser '917 patent, the pond of deaerated stock occupies space at the bottom of the vacuum receiver. The vacuum receiver, therefore, must be large enough to contain both the pond and a headspace of adequate size above the pond. The vacuum receiver may be completely filled with liquid stock during abnormal operation or during hydrostatic testing. The receiver-supporting structure must normally be designed to support the weight of the receiver in this completely-filled condition, and such weight is directly proportional to the volume of the receiver. Elimination of the pond within the vacuum receiver, or substantial reduction in the size of such pond, would permit use of a smaller, less expensive receiver and would also permit use of a less expensive supporting structure.
Moreover, deaeration apparatus according to the Kaiser '917 patent generally is not maintained in operation when operation of the processing machine is temporarily interrupted. During such a temporary shutdown of the processing machine, outflow of stock through the main outlet at the bottom of the main portion of the receiver is interrupted. If the deaeration apparatus were maintained in operation, all of the deaerated stock would have to exit from the receiver via the recycle line. It is costly to provide the recycle line with sufficient capacity for such flow and to provide the recirculation piping needed to prevent undesirable stagnation of stock contained in the piping leading from the main outlet of the receiver to the processing machine during such a shutdown.
Deaeration systems generally require substantial time to reach equilibrium after restarting. If operation of the deaeration system is interrupted during a temporary shutdown of the processing machine and then restarted when the machine is restarted, the machine may not make useful product until the deaeration system reaches equilibrium. Continued operation of the deaeration system during temporary machine shutdowns minimizes the loss of productive time and materials upon restarting of the processing machine. Thus, a deaeration system which could be more economically provided with capacity for continued operation during temporary processing machine shutdowns would be most desirable.